The present invention relates to a device to control and damp the subsynchronous vibrations of centrifugal compressors.
In general, centrifugal compressors are machines which impart to a compressible fluid a pressure which is greater than that at which it receives the fluid, providing the latter with the energy necessary for this pressure increase with one or more impellers or rotors disposed in series, which are provided with radial blades and are activated at a high speed, by means of a motor which in general is keyed onto the compressor shaft itself.
Typically the centrifugal compressors fulfill a great variety of uses with requirements for high capacities and medium-low pressures, such as in refrigeration systems, in the petrochemicals industry, for example ethylene systems, catalytic cracking systems, and units for compression of CO2 in urea systems, in the energy industry, such as systems for LPG, for oxygen and the units for compression and release to the gas pipeline service. The power levels installed are in general substantial and the pressures are as much as 40 bars.
A general drawing of the structure of a centrifugal compressor is illustrated in FIG. 1, in order to illustrate the technical problem to which the present invention relates. The blades of the rotor 1, i.e. of the rotary part, co-operate with a stator 2, i.e. a fixed part which is interposed between the adjacent impellers and through which there passes the shaft 3, which connects and supports the impellers. The blades of the rotor can have various shapes, and in FIG. 1 impellers 4 are shown which have as blades closed channels 5, which make possible a structure with greater strength and improved guiding of the flows. The fixed part or stator consists of a body 6 which surrounds the outlet openings of the closed channels 5 of the impeller 4 and is subdivided into a plurality of deflector vanes 8 which, in the various stages, co-operate with the blades or channels of the preceding impeller in order to receive the fluid at a high speed, transform into pressure energy part of the speed which is impelling the fluid, and deflect and convey it into the innermost part of the blades of the successive compression impeller. The pressure of the fluid thus increases from stage to stage until it reaches its final value.
In the axial direction, there is generation of pressure differential between the various stages, which thus requires insertion on the shaft 3 of the rotor 1 of a system of seals between the rotor and stator of each stage, which limits as far as possible the phenomenon of reflux of the compressed fluid to the preceding stages, in order to maintain the compression performance at appropriate values.
As the pressure value progresses from upstream in the downstream direction, on the rotor body there is thus generation of axial and radial forces which must be equilibrated and balanced both statically and dynamically, owing to the presence of the inevitable irregularities over a period of time in the system as a whole.
One of the most sought-after characteristics of the rotors of centrifugal compressors, and of rotary machines which operate at high speed and with fluids at high pressures, is their dimensional stability even in the presence of operating variations caused by temporal irregularities of flow upstream or downstream or of the real density or pressure of the gas on which work is taking place. The forces which are generated by these irregularities of density or pressure of the fluid give rise, in the labyrinths of the seals at the ends of the machines and of the passages from stage to stage, to subsynchronous vibrations on the rotor which are extremely detrimental to the operation and efficiency of the machine. This tangential course of the fluid in the machine or wherever there is a part which rotates relative to a fixed part, with limited tolerances, such as in the sealing units or balancing pistons, gives rise to destabilizing forces and vibrations which must be balanced and damped.
The dynamic performance of the rotor must therefore always be controlled and included at the design stage of the installation for each specific application, with reference to its stability in terms both of flexure and torsion. For this reason, and taking into consideration the fact that the negative influence of these vibrations on the rotor depends directly on the flexure and torsion induced on it, as a solution there is a tendency to increase the rigidity of the rotor and decrease its flexibility, both by using shafts with a large diameter and by shortening their section with free flexure, by reducing as far as possible the pitch between the support bearings.
A consequence of the step of increasing the diameter of the shaft and shortening the pitch of the supports consists of the complication of the problem of the seals on the rotary shaft, and especially the seals to be produced between the stages with the highest pressure of the compressor and the surrounding atmosphere. An increase in the diameter thus corresponds to a greater linear development of the play to be controlled, thus limiting the flows of the compressed fluid which escapes via the play of the seals.
In recently designed compressors, in order to damp and restrict the phenomena of vibration, in addition to the normal bearings for support, and to withstand the axial thrust, balancing pistons are inserted in the end parts of the drive shaft of the compressor.
With reference to the play which exists between the elements which constitute the seals or the balancing pistons and which are interposed between the impeller or rotor part and the fixed part or stator, account must be taken of the fact that this play is necessary and its dimensions must be appropriate, taking into consideration both the foreseeable deformations caused by the mechanical stresses and the expansions/contractions caused by the temperature variations. When evaluating the flexure deformations of the various parts, account must also be taken of the parts own frequencies and of the modes of natural vibration in terms of flexure and torsion at the various speeds of the rotor. On the other hand as far as the thermal ranges are concerned, account must be taken of the dimensional variations caused by the temperature during the transient phases, in which the stator and rotor can heat up and cool down, and thus expand or contract, with a greater or lesser speed relative to one another.
Excessively narrow play causes friction, heating and wear which detracts from the efficiency, the factor of use and the technical service life of the machine. Excessive play detracts from the performance of the machine.
The balancing pistons which are placed at the ends of the compressor shafts can be of various types, with bodies provided with cavities or venting openings, and are used to create an action of damping, which assists control of the subsynchronous vibrations of the compressor. In recent times, in the balancing pistons for compressors, considerable preference has been given to the use of sealing bodies which are commonly known as honeycomb by persons skilled in the art. This type of balancing piston is shown by way of example in the detail in FIG. 1A.
On the rotor 1 side, on the body of the shaft 3, there is keyed a cylindrical body 10 which acts as a balancing piston and separates a space 11 inside the compressor in which a greater pressure exists, from a space 12 in which a lower pressure exists. The outer cylindrical surface 13 of the body 10 is smooth and is parallel to the axis of the shaft 3. On the stator 2 side, at the axial correspondence of the balancing piston 10, there is fitted a hollow cylindrical structure 15 which has in its interior a seal with an annular body 16 provided with a large number of small hollow cells, in the form of a honeycomb. The honeycomb seal 16 can be made of metal material in a known manner, and can be secured to the structure 15 by conventional means, for example by being brazed. Between the outer surface 13 of the balancing piston 10 and the surface of the honeycomb seal 16 opposite it, play 17 of a substantially constant value is normally maintained for the entire length affected by the piston 10.
The effect of damping of the subsynchronous vibrations of a compressor caused by this piston provided with a honeycomb seal is generally attributed to the small cell structure, which acts such as to change the acoustic response of the discharge of the fluid from the high-pressure space to the low-pressure space within the play 17 between the rotor part and the stator part. This effect can be attributed to the fact that during its discharge the fluid in fact encounters a succession of wider and narrower passages, in which it repeatedly slows down and accelerates, gradually losing energy and pressure, damping the vibrations, and losing energy by Bernoulli effect.
The object of the present invention is a balancing piston device which is more efficient in damping the vibrations of the rotors for centrifugal compressors, such as to improve their effects of damping and adaptability to a greater number of industrial applications.
In order to illustrate more clearly the characteristics and advantages of the present invention, the latter is described with reference to a typical embodiment which is illustrated in FIGS. 1, and 2-5 by way of non-limiting example.